Electrical fuses (eFuses) are used in the semiconductor industry to implement array redundancy, field programmable arrays, analog component trimming circuits, and chip identification circuits. Once programmed, the programmed state of an electrical fuse does not revert to the original state on its own, that is, the programmed state of the fuse is not reversible. For this reason, electrical fuses are called One-Time-Programmable (OTP) memory elements.
The mechanism for programming an electrical fuse is electromigration of a metal semiconductor alloy induced by an applied electrical field and an elevated temperature on a portion of the electrical fuse structure. The metal semiconductor alloy is electromigrated under these conditions from the portion of the electrical fuse structure, thereby increasing the resistance of the electrical fuse structure. The rate and extent of electromigration during programming of an electrical fuse is dependent on the temperature and the current density at the electromigrated portion.
An electrical fuse typically comprises an anode, a cathode, and a fuselink. The fuselink is a narrow strip of a conductive material adjoining the anode and cathode. During programming of the electrical fuse, a positive voltage bias is applied to the anode and a negative voltage bias is applied to the cathode. As electrical current flows through the fuselink having a narrow cross-sectional area, the temperature of the fuselink is elevated. A high current density combined with the elevated temperature at the fuselink facilitates electromigration of the conductive material, which may comprise a metal silicide.
A typical prior art electrical fuse employs a stack of a gate dielectric, a polysilicon layer, and a metal silicide layer. Under electrical bias through the electrical fuse, the metal silicide layer provides an initial current path since a typical metal silicide material has a conductivity at least one order of magnitude greater than the conductivity of even the most heavily doped polysilicon material. As the metal silicide material electromigrates, the electrical current path formed by the initial metal silicide layer is broken. Further, the high temperature that the metal silicide layer generated prior to completion of electromigration contributes to dopant electromigration in the polysilicon layer underneath, causing depletion of the dopants in the polysilicon layer in a programmed prior art electrical fuse. A programmed electrical fuse attains a high enough resistance so that a sensing circuit may detect the programmed electrical fuse as such. Thus, the prior art electrical fuse containing a vertically abutting stack of the gate dielectric, the polysilicon layer, and the metal silicide layer provides an OTP memory element without introducing any additional mask level or any extra processing steps.
Despite the general improvement in the distribution of post-programming resistance of electrical fuses through the use of electromigration mode programming, not all fuses produce a post-programming resistance distribution with high resistance values even in an electromigration mode. The distribution of the resistance of programmed fuses is also dependent on the design of fuses as well; some producing more low resistance values for programmed fuses, while some others produce less low resistance values.
In view of the above, there exists a need for an electrical fuse structure providing good programming characteristics including high post-programming resistance, and methods of manufacturing the same.